Multistage thin film toning system for developing charged images

ABSTRACT

During development of the electrostatic image, fresh toner is applied to the electrostatically charged paper by removing the depleted boundary layer of toner. The moving paper establishes a drag force on the toner forming a boundary layer which acts as a surface barrier preventing the migration of new toner particles to the paper. The development of the electrostatic image is completed in stages, each stage containing a toning mechanism to apply the toner and a squeegee mechanism to remove the toner boundary layer. In the preferred embodiment, the initial squeegee mechanism is a stationary bar, and the final squeegee mechanism is a roller squeegee partially formed by a paper drive roller.

' United States Patent Kase et al. Dec. 16, 1975 MULTISTAGE THIN FILM TONING 3,474,757 10/1969 Dreher 118/411 S S FOR DEVELOPING CHARGED 3,664,298 5/1972 Giaimo, Jr. 118/637 IMAGES 3,690,297 9/1972 Dentch et al. 118/410 3,788,844 1/1974 Sato et a1 117/37 LE [75] Inventors: Heino Kase, Walnut Creek; Edward W- M sal'atogai Survfat Primary Examiner--Mervin Stein Kmkar, Stanford, of Cahf- Assistant Examiner-Steven Hawkins [73] Assignee: Varian Associates, Palo Alto, Calif. Attorney Agent or Firmstanley Cole;

Stoddard [22] F1led: May 1, 1972 [21] Appl. N0.: 249,258 [57] ABSTRACT During development of the electrostatic image, fresh [52] US. Cl 346/74 ES; 118/637; 118/DIG 23; toner is applied to the electrostatically charged paper 427/15 by removing the depleted boundary layer of toner. [51] Int. C1. BOSB 5/02 T moving p p r a lishes a r g force on h [58] Field Of Search 1 17/37 LE; 118/429, 637, toner forming a boundary layer which acts as a surface 1 18/DIG 23 barrier preventing the migration of new toner particles to the paper. The development of the electrostatic [56] References Cited image is completed in stages, each stage containing a UNITED STATES PATENTS toning mechanism to apply the toner and a squeegee 1 mechanism to remove the toner boundary layer. In the gla preferred embodiment, the initial squeegee mecha- 3:196:832 7/1965 2111...Q:I.:::::::.......I:I 118/DlG: 23 is a Stationary-bar and the final Squeegee mecha' 3,344,729 10/1967 Kitrosser l18/D1G. 23 18 a roller Squeegee Partially formed y a P p 3,345,927 10/1967 Takats ll8/D1G. 23 dnve roller- 3,37l,651 3/1968 Johnson et a1 118/D1G. 23 7 Cl 4 D F. 3,470,848 10/1969 Dreher 118/429 raw'ng gums t it l I 22 E473 j-l: 58 14 1 f: T-ff- 36 16 FILTER 1 4 62 A 4 F3 n Z -50 U.S. Patent Dec. 16,1975 Sheet 1 of2 3,927,409

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2' TIME OF TONING EXPOSURE ($50.)

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US. Patent Dec. 16, 1975 Sheet20f2 3,927,409

FIG.4

MULTISTAGE THIN FILM TONING SYSTEM FOR DEVELOPING CHARGED IMAGES FIELD OF THE INVENTION This invention relates to multistage toning and more particularly to thin film liquid toning.

DESCRIPTION or THE PRIOR ART,

The toning process is initiated by exposing the charge image to charged toner particles dispersed as a colloid in an insulating liquid diluent or carrier liquid. Mutual electrostatic attraction causes the toner particles to deposit on the charge image. This attraction force is directly proportional to the square of the distance between the charge image and the charge toner particle. The toner particles deposited on the charge image adhere thereto to form a permanentvisible image. The toner density of the final image and development time required are a function of the charge intensity on the paper, the magnitude of the charge on the toner particles, the viscosity and temperature of the diluent, and the concentration of toner particles in the diluent.-

The toning process is initially vigorous and gradually decays because the net charge on the paper decreases due to the neutralizing effect of the deposited charged toner particles. The force of attraction between the toner particles and the paper is correspondingly reduced. A further decrease in the toning activity is caused by depletion of the toner in the boundary layer formed immediately adjacent to the paper. The toner particles more remote from the paper are attracted with less force and have a greater distance to travel.

The time required for a toner particle to migrate to the paper from a distance X is given by the following cubic equation:

8 1r r p. E,J x t T where t Time r radius of carbon particle u viscosity of toner E 885x coulombs/nt-m x distance of the toner particle from the paper Q charge on the paper e charge on toner particles FIG. 1 shows an exponential curve of toning completed plotted against time representing the above mathematical relationship. The first derivative of this curve the rate of toner activity or toner particle transfer is initially very great, and progressively decreases approaching zero as the toning approaches 100%. Under the toning conditions of the preferred embodiment, 3% seconds are required to form an 80% toned image. The decrease in toner rate is caused by a gradual decrease in electrostatic forces of attraction due to the neutralization of the charge image, and to gradual depletion of the toner in the thin film or boundary layer proximate the moving web. This laminar flow caused by relative motion between the paper and the toner, prevents fresh toner from approaching the paper.

SUMMARY OF THE INVENTION It is therefore an object of this invention to: provide a multistage toning system which maintains a high toner transfer rate; provide a toning multistage system permitting an increased web speed to toning time ratio; provide a toning multistage system which minimizes the time between charge image formation and visible inspection; and provide a multistage toning system wherein the motion of the web provides the squeegee action required to remove the depleted toner boundary layer.

BRIEF DESCRIPTION OF THE DRAWING Further objects and advantages of the present multistage toning system and the operation of the squeegee technique will become apparent from the following detailed description taken in conjunction with the drawing in which:

FIG. 1 is a curve of percent toning versus exposure time of a single stage prior art toning system showing the effects of toner depletion and laminar flow;

FIG. 2 is a schematic view of the present multistage toning apparatus;

FIG. 3 is a pictorial view of the toning station of FIG. 1 broken away to show the flow equalizing structure therein; and

FIG. 4 is a curve of percent toning versus time of a two stage toning device of FIG. 2 showing the increase of toning rate brought about by squeegeeing of the boundary layer and applying fresh toner.

FIG. 2 shows an electrostatic recorder 10 which forms a charge image on web 12 by electrical interaction between a recording head 14 containing styli l6 and a backplate l8 spaced therefrom to permit passage of the web 12. Drive roller 20 moves web 12 with the charge image formed thereon along a path of motion past a toning station 22 which includes a first toning slot 24, a squeegee bar 26 and a second toning slot 28. A toning pump 30 pumps toner from toner supply 32 through toner filter 34 and into a station reservoir 36 in toning station 22. The incoming toner passes through a flow equalizing perforated baffle 38 described in more detail in FIG. 3. The toner moves upward in planar columns 40 and 42 and onto web 12 through first toning slot 24 and second toning slot 28 respectively, The toner is supplied to the moving web 12 in a thin film 41 which initially develops the charge image very actively. The toning activity decreases as thin film 41 approaches toner depletion. Depleted toner film 41 is removed from web 12 by squeegee bar 26. Fresh toner is applied to the squeegeed partially toned charge image by second toning slot 28 forming thin toner film 43. Once again the toning process is initiated with vigor and gradually decreases with toner depletion. This second toner film 43 is removed from web 12 by the pressing action of a squeegee roller 44 mounted proximate drive roller 20. The twice toned paper web 12 is then directed out of recorder 10 for visible inspection (not shown). Toning is permitted in each stage only during the initial period when the toning rate is high. After the charge image exposed to each toning stage reaches about 50% toned, the toning rate has slowed down greatly and continued toning is inefficient. After about 50% toning in any stage, the toner film should be squeegeed off and fresh toner applied.

Toner film 41 removed from web 12 by squeegee bar 26 flows down squeegee bar 26 into a toner collection trough 46. Toner film 43 removed from web 12 by squeegee roller 44 flows downward into a toner pan 48 which also collects toner from collection trough 46. The toner in pan 48 drains through a toner return line 3 50 back to toner supply 32.

A guide roller 52 is provided to contour web 12 around the curved surface of writing head 14 and to seal web 12 against a lip 54 which forms the leading edge of first toning slot 24. The sealing between web 12 and lip 54 prevents toner from flowing down the outside of toning station 22, and establishes the toner flow parallel to and in the same direction as web 12 motion to promote the formation of thin film 41. Thin film 41 is limited to less than mils in thickness due to the viscosity and surface tension of the toner, and is equivalent to a 5 mil drag free boundary layer moving along with the web. The boundary layers of the prior art were much thinner due to fluid drag between the moving boundary layer and the remaining stationary toner. The differential velocities thus established between the web and the toner inhibited toner particle migration to the web. The present drag free boundary layer moves in unison with the web and the differential velocity problem is avoided. The only limitation on drag free boundary layers 41 and 43 is toner depletion which occurs in a much thinner thickness than 5 mils in the short toner exposure time of FIG. 2.

FIG. 3 shows in greater detail the internal structure of station reservoir 36 which establishes a uniform toner flow along the length of toning slots 24 and 28. The toner enters reservoir 36 at entrance ports 60 into a lower reservoir chamber 62. Perforated baffle 38 separates lower chamber 62 from an upper chamber 64 and uniformly distributes the incoming toner flow along the entire length of toning station 22. Upper reservoir chamber 64 forms planar toner columns 40 and 42 which flow out toning slots 24 and 28. The uniform flow is established by breaking up the flow pattern issuing from ports 60. The flow pattern is disrupted by the multiple changes in direction caused by baffle 38 and the space apertures therein. Prior art toner spraying devices did not uniformly spray the toner which caused light and dark areas in the resulting toned image. These prior art devices are analogous to lawn sprinklers it is difficult to obtain uniform watering.

In the preferred embodiment, web 12 is moved at a rate of 1.67 inches per second. First toning slot 24 is spaced about 0.9 inches from squeegee bar 26 and second toning slot 28 is spaced about 1.6 inches from the contact point between squeegee roller 44 and drive roller 20. The toning stations have a toning exposure time of about 0.54 and 0.96 seconds respectively. FIG. 4 shows a curve of percent toning versus time of a two stage toning station. At time zero web 12 is initially toned at toning slot 24. At .54 the partially depleted toner is removed from web 12 by squeegee bar 26. At this point web 12 is about 35% toned. Fresh toner is immediately applied to web 12 through second toning slot 28. At 1.50 the squeegee action between rollers 44 and press out the toner terminating the toning action. At 1.50 the toning is 80% complete. The total time elapsed for 80% toning by the present multistage toning system is only 1.50 seconds. FIG. 1 shows 3.25 seconds required to accomplish 80% toning with a single stage toner of the prior art.

In the preferred embodiment the toning slots are about 60 mils wide and about 14.25 inches long. The uniform flow rate is established by 7 apertures 0.125 inches in diameter provided on baffle 38 spaced about 2 inches apart. The preferred toner flow rate is about 90 cubic inches per minute. A 3 mil toner film on a l4 inch web traveling l.67 inches per second requires 7 cubic inches of toner per minute, and it is essential that pump 30 supply at least this flow quantity to maintain a uniform film. The toning rate of thin films 41 and 43 are independent of the toner flow rate because the thickness of films 41 and 43 are determined by the viscosity and surface tension of the toner. Variations in toner flow may change the amount of excess toner which flows down the outside walls of toning station 22; but flow variations will not affect the thickness of toner films 41 and 43 so long as a critical minimum flow rate is maintained.

Clearly, various changes may be made in the structure and embodiments shown herein without departing from the concept of the present invention. For example, additional toning stages may be employed for accomplishing the toning or greater in an even shorter time than the illustrated dual stage toning device. Two bar type squeegees may be employed instead to replace the squeegee roller which cooperates with drive roller 20. The bar squegees are less expensive to manufacture and maintain but the squeegee rollers are non-abrasive and assist in driving web 12.

We claim as our invention:

1. A recording device for forming a charge image on the charge retentive surface of a recording medium and toning the charge image with toner particles dispersed in a carrier liquid, comprising the combination:

means for moving the recording medium relative to the recording device along a path of motion;

means for forming a charge image on the charge retentive surface of the recording medium;

a plurality of liquid toner applying means spaced from one another along the path of motion of said recording medium, each of said toner applying means arranged to apply a thin film of toner to the charge image on the recording medium;

a. liquid toner squeegee means positioned between each of the spaced toner applying means to remove the thin film of toner from the charge retentive surface before the toner becomes substantially depleted of toner particles; and

final liquid toner squeegee means positioned subsequent to the last toner applying means along the path of motion to remove the thin film of toner applied by the last toner applying means.

2. The recording device of claim 1, wherein the charge image is formed on the underneath side of the recording medium and the plurality of toner applying means flow the toner upward into contact with the underneath side of the recording medium in sufficient quantity to form a thin toner film the thickness of which is determined by the surface tension of the toner independently of the toner flow rate.

3. The recording device of claim 2, wherein the plurality of toner applying means are formed by a slot opening at the top of an upwardly directed channel extending across the recording medium generally transverse of the direction of web motion, the leading edge of the slot opening extending slightly higher than the trailing edge of the slot opening to form a bar type squeegee which removes the thin film of toner from the recording medium immediately prior to applying toner thereto from the slot opening.

4. The recording device of claim 3, wherein the leading edge of the initial toner slot opening extends higher than the trailing edge thereof to engage the recording medium and prevent toner leakage.

6 scrape the thin toner film from the recording medium which is held in tension against the bar type squeegees by the drive roller and squeegee roller.

7. The recording device of claim 1, wherein the thin film of toner applied by each applying means is removed from the charge retentive surface by the subsequent squeegee means before the charge image available at the time of each toner application becomes 50 percent toned. 

1. A recording device for forming a charge image on the charge retentive surface of a recording medium and toning the charge image with toner particles dispersed in a carrier liquid, comprising the combination: means for moving the recording medium relative to the recording device along a path of motion; means for forming a charge image on the charge retentive surface of the recording medium; a plurality of liquid toner applying means spaced from one another along the path of motion of said recording medium, each of said toner applying means arranged to apply a thin film of toner to the charge image on the recording medium; a. liquid toner squeegee means positioned between each of the spaced toner applying means to remove the thin film of toner from the charge retentive surface before the toner becomes substantially depleted of toner particles; and final liquid toner squeegee means positioned subsequent to the last toner applying means along the path of motion to remove the thin film of toner applied by the last toner applying means.
 2. The recording device of claim 1, wherein the charge image is formed on the underneath side of the recording medium and the plurality of toner applying means flow the toner upward into contact with the underneath side of the recording medium in sufficient quantity to form a thin toner film the thickness of which is determined by the surface tension of the toner independently of the toner flow rate.
 3. The recording device of claim 2, wherein the plurality of toner applying means are formed by a slot opening at the top of an upwardly directed channel extending across the recording medium generally transverse of the direction of web motion, the leading edge of the slot opening extending slightly higher than the trailing edge of the slot opening to form a bar type squeegee which removes the thin film of toner from the recording medium immediately prior to applying toner thereto from the slot opening.
 4. The recording device of claim 3, wherein the leading edge of the initial toner slot opening extends higher than the trailing edge thereof to engage the recording medium and prevent toner leakage.
 5. The recording device of claim 1, wherein a drive roller is provided for moving the recording medium, and wherein the final squeegeeing means is a squeegee roller engaging the recording medium against the drive roller for moving and maintaining tension in the recording medium and for pressing the toner film off of the recording medium.
 6. The recording device of claim 5, wherein the plurality of squeegeeing means positioned between the toner applying means are bar type squeegees which scrape the thin toner film from the recording medium which is held in tension against the bar type squeegees by the drive roller and squeegee roller.
 7. The recording device of claim 1, wherein the thin film of toner applied by each applying means is removed from the charge retentive suRface by the subsequent squeegee means before the charge image available at the time of each toner application becomes 50 percent toned. 